1. The Field of the Invention
The present invention relates to the field of semiconductor devices including optical amplifiers and semiconductor lasers. More particularly, the present invention relates to systems and methods for tuning a Distributed Bragg Reflector (DBR) mirror stack in a semiconductor device.
2. Related Technology
A linear optical amplifier is a semiconductor device that includes an active region. An optical signal incident to the active region is amplified and output by the optical amplifier. The active region is located between a p-type semiconductor layer and an n-type semiconductor layer. In one example, the p-type and n-type semiconductor layers include Distributed Bragg Reflector (DBR) layer stacks. As a result, the optical amplifier has a structure similar to a vertical cavity surface emitting laser (VCSEL). The length of the active region in the direction of the optical signal being amplified is longer.
When a potential is applied across the active region, the VCSEL begins lasing at a threshold current. The lasing field acts as a ballast and stabilizes the gain of the optical amplifier and makes the gain more linear. Thus, the optical amplifier and the VCSEL share the same gain medium. The optical fields of the optical amplifier and the VCSEL, however, propagate in orthogonal directions. The VCSEL may emit light out of the top (or bottom) surface, while the amplified optical signal emits from the side of the device.
The linear optical amplifier, which effectively combines a VCSEL and a semiconductor optical amplifier, provides linear gain. The gain, however, may be adversely affected by the DBR layers in the DBR stacks. The DBR layers form a multimode slab waveguide that is parallel to the active region. As a result, light can be lost from the active region due to evanescent coupling. The mode interaction between the active region and the DBR layers may also be wavelength dependent and may result in a loss in gain of the amplifier near the coupling wavelength.
One consequence of the mode interaction between the active region and the DBR layers is that the gain spectrum may exhibit areas where the gain dips, referred to as channel drops. If the gain spectrum exhibits channel drops, then the optical amplifier is usable over a relatively narrow bandwidth. The presence of gain drops can lessen the usefulness of the optical amplifier. Nearly constant gain over the wavelengths of interest is desired.